1. Field of the Invention
The present invention relates to an image processing apparatus that is mounted in a digital color copying machine or other equipment. More specifically, the present invention relates to an interline correction process for correcting a misregistration among red, green and blue colors in the secondary scanning direction of a contraction type color CCD sensor. The present invention also relates to a chromatic aberration correction for correcting a phase shift in the primary scanning direction among image data of red, green and blue colors due to a chromatic aberration of a lens system.
2. Description of the Prior Art
An image reader portion of a color copying machine or other equipment usually has a contraction type color CCD sensor for reading information of an original image that is contracted and projected through an optical system, as described in Japanese unexamined patent publication No. 9-261491(A) for example. This is a common configuration since it is advantageous to the cost. The contraction type color CCD sensor has a structure shown in FIG. 10 in which three element arrays of red, green and blue colors are arranged in parallel with each other at a predetermined distance d in the secondary scanning direction. Each of the element arrays has plural elements of red, green or blue colors aligned in the primary scanning direction.
When using the above-mentioned color CCD sensor for reading an image, a time-base shift, i.e. a phase shift among image data of red, green and blue colors obtained by the CCD sensor is generated because of a position shift (by a distance d) between the element arrays of red, green and blue colors in the secondary scanning direction. The secondary scanning direction is a direction in which an original image and the CCD sensor move relatively to each other.
In order to correct the phase shift among red, green and blue colors (hereinafter, referred to as a misregistration), red image output data that are generated first are delayed by the time corresponding to the distance 2d (e.g., eight lines), and green image output data that are generated after the red image output data are delayed by the time corresponding to the distance d (e.g., four lines), so that the phases of the red and green image output data are adjusted to the phase of the blue image output data that are generated at last.
For example, in a color copying machine that has reducing and enlarging function, if the scaling ratio for contracting and projecting an original image is changed in such a way that a scanning speed is changed in the secondary scanning direction, the phase shift among red, green and blue colors cannot be an integral multiple of a line, but can have a remainder (a fraction). In this case, the phase shift among red, green and blue colors should be corrected as precisely as possible by an interpolation process. Namely, if the position after the correction is between the lines, the density of each color of the position is determined as a weighted average of the densities of the lines.
However, there is a case where a reproducibility of a black fine line is deteriorated by the above-mentioned correction of the fraction by the interpolation process in the correction of the phase shift among red, green and blue colors using a color CCD sensor. It is considered that the above-mentioned phenomenon is caused by an imbalance among reading characteristics of red, green and blue colors that can be generated by the above-mentioned interpolation process when the black fine line is contracted and projected in e.g., one dot width.
Therefore, the densities of colors except the reference color of the correction are decreased. As a result, the black fine line becomes greeny or reddish, so that the reproducibility of the black fine line is deteriorated.
It is generally known that the color deviation (a phase shift among colors) becomes larger in the periphery farther from the optical axis of the lens system due to a chromatic aberration generated by different refractive indexes of different wavelengths of light. This color deviation due to the chromatic aberration in the color copying machine can be suppressed within a small range less than one dot by improving the lens system or other means, but cannot be eliminated completely.
The color deviation due to the chromatic aberration affects particularly the reproducibility of black letters (including a black fine line) badly. A usual color copying machine determines black letter portions from image data and performs correction process such as edge emphasizing so that the reproducibility of the black letter portions can be improved. Various methods for determining black letter portions are proposed. Each method discriminates between an achromatic color and a chromatic color in accordance with image data of red, green and blue colors. For example, the difference between the maximum and minimum values of the image data of red, green and blue colors is calculated to be a chroma value. This chroma value is compared with a threshold value so as to discriminate between an achromatic color and a chromatic.
However, if the above-mentioned color deviation due to the chromatic aberration increases, the difference between the maximum and minimum values of the image data of red, green and blue colors, i.e., the chroma value becomes larger than the threshold value even in the case where a black letter is read, resulting in a failure to recognize the achromatic color. Consequently, the black letter cannot be recognized as a black letter portion, and a correction process such as edge emphasizing is not performed for the black letter, resulting in a deterioration of the reproducibility of the black letter.
In order to suppress the deterioration of the reproducibility of the black letter due to the chromatic aberration, the correction process proposed in the above-mentioned Japanese unexamined patent publication No. 9-261491(A) prepares plural kinds of filters for shifting the red image data and the blue image data in the opposite direction to each other with respect to the green image data by the same quantity (less than one dot in the primary scanning direction). The red image data obtained by red color light with long wavelength and the blue image data obtained by blue color light with short wavelength have phase shift directions opposite to each other with respect to the green image data obtained by the green light, and their phase shift directions are opposite to each other in both sides of the primary scanning direction.
Therefore, plural filters having different directions and quantities of phase shift are prepared for passing simultaneously the image data obtained by reading an original image. Then, the difference between the maximum and minimum values of the image data obtained from the filters, i.e., the chroma value is calculated. The minimum chroma value is adopted as the corrected chroma value, which is used for determining the black letter portion as explained above.
Since plural filters having appropriate shift quantities within a possible range are prepared, black letter portions cannot be determined incorrectly to have a chromatic color even if a phase shift due to the chromatic aberration is generated at the end of the primary scanning direction. Thus, the correction process such as edge emphasizing is performed properly, so that the reproducibility of the black letter portion can be improved.
Though the correction process of the chromatic aberration is performed uniformly over the entire area in the primary scanning direction, both edge portions in the primary scanning direction may be parts in which the chromatic aberration can affect the most. However, since the above-mentioned correction process of the chromatic aberration is also performed for the center portion where the influence of the chromatic aberration is little, a green color line is changed to have a low chroma, for example. As a result, the edge portion of the green line can be determined incorrectly to be the edge portion of the black line. This incorrect determination is required to be improved.
The filter used for the above-mentioned correction process shifts the red image data and the blue image data in the opposite direction to each other and by the same quantity. However, the quantity of the phase shift due to the chromatic aberration varies for each color in accordance with characteristics of the lens system or the resolution of an image sensor.
Furthermore, the above-mentioned correction process does not correct the image data of each color, but merely corrects the influence of the chromatic aberration on the determination of the chroma value for sensing black letters.